Reading brain signals without inserting electrodes

8 July 2009

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Experimental devices that read brain signals have helped paralyzed
people use computers and may let amputees control bionic limbs. But
existing devices use tiny electrodes that poke into the brain. Now, a
University of Utah study shows that brain signals controlling arm
movements can be detected accurately using new microelectrodes that sit
on the brain but don’t penetrate it [1].

“The unique thing about this technology is that it provides lots of
information out of the brain without having to put the electrodes into
the brain,” says Bradley Greger, an assistant professor of
bioengineering and coauthor of the study. “That lets neurosurgeons put
this device under the skull but over brain areas where it would be risky
to place penetrating electrodes: areas that control speech, memory and
other cognitive functions.”

For example, the new array of microelectrodes someday might be placed
over the brain’s speech centre in patients who cannot communicate
because they are paralyzed by spinal injury, stroke, Lou Gehrig’s
disease or other disorders, he adds. The electrodes would send speech
signals to a computer that would covert the thoughts to audible words.

For people who have lost a limb or are paralyzed, “this device should
allow a high level of control over a prosthetic limb or computer
interface,” Greger says. “It will enable amputees or people with severe
paralysis to interact with their environment using a prosthetic arm or a
computer interface that decodes signals from the brain.”

The findings represent “a modest step” toward use of the new
microelectrodes in systems that convert the thoughts of amputees and
paralyzed people into signals that control lifelike prosthetic limbs,
computers or other devices to assist people with disabilities, says
University of Utah neurosurgeon Paul A. House, the study’s lead author.

“The most optimistic case would be a few years before you would have
a dedicated system,” he says, noting more work is needed to refine
computer software that interprets brain signals so they can be converted
into actions, like moving an arm.

An advance over the penetrating Utah Electrode Array

Such technology already has been developed in experimental form using
small arrays of penetrating electrodes that stick into the brain. The
University of Utah pioneered development of the 100-electrode Utah
Electrode Array used to read signals from the brain cells of paralyzed
people. In experiments in Massachusetts, researchers used the small,
brain-penetrating electrode array to help paralyzed people move a
computer cursor, operate a robotic arm and communicate.

Meanwhile, researchers at the University of Utah and elsewhere are
working on a $55 million Pentagon project to develop a lifelike bionic
arm that war veterans and other amputees would control with their
thoughts, just like a real arm. Scientists are debating whether the
prosthetic devices should be controlled from nerve signals collected by
electrodes in or on the brain, or by electrodes planted in the residual
limb.

The new study was funded partly by the Defense Advanced Research
Projects Agency’s bionic arm project, and by the National Science
Foundation and Blackrock Microsystems, which provided the system to
record brain waves.

House and Greger conducted the research with Spencer Kellis, a
doctoral student in electrical and computer engineering; Kyle Thomson, a
doctoral student in bioengineering; and Richard Brown, professor of
electrical and computer engineering and dean of the university’s College
of Engineering.

Microelectrodes on the brain may last longer

Not only are the existing, penetrating electrode arrays undesirable
for use over critical brain areas that control speech and memory, but
the electrodes likely wear out faster if they are penetrating brain
tissue rather than sitting atop it, Greger and House say. Nonpenetrating
electrodes may allow a longer life for devices that will help disabled
people use their own thoughts to control computers, robotic limbs or
other machines.

“If you’re going to have your skull opened up, would you like
something put in that is going to last three years or 10 years?” Greger
asks.

“No one has proven that this technology will last longer,” House
says. “But we are very optimistic that by being less invasive, it
certainly should last longer and provide a more durable interface with
the brain.”

The new kind of array is called a microECoG — because it involves
tiny or 'micro' versions of the much larger electrodes used for
electrocorticography, or ECoG, developed a half century ago.

For patients with severe epileptic seizures that are not controlled
by medication, surgeons remove part of the skull or cranium and place a
silicone mat containing ECoG electrodes over the brain for days to weeks
while the cranium is held in place but not reattached. The large
electrodes — each several millimeters in diameter — do not penetrate the
brain but detect abnormal electrical activity and allow surgeons to
locate and remove a small portion of the brain causing the seizures.

ECoG and microECoG represent an intermediate step between electrodes
that poke into the brain and EEG (electroencephalography), in which
electrodes are placed on the scalp. Because of distortion as brain
signals pass through the skull and as patients move, EEG isn’t
considered adequate for helping disabled people control devices.

The regular-size ECoG electrodes are too large to detect many of the
discrete nerve impulses controlling the arms or other body movements. So
the researchers designed and tested microECoGs in two severe epilepsy
patients who already were undergoing craniotomies.

The epilepsy patients were having conventional ECoG electrodes placed
on their brains anyway, so they allowed House to place the microECoG
electrode arrays at the same time because “they were brave enough and
kind enough to help us develop the technology for people who are
paralyzed or have amputations,” Greger says.

The researchers tested how well the microelectrodes could detect
nerve signals from the brain that control arm movements. The two
epilepsy patients sat up in their hospital beds and used one arm to move
a wireless computer “mouse” over a high-quality electronic draftsman’s
tablet in front of them. The patients were told to reach their arm to
one of two targets: one was forward to the left and the other was
forward to the right.

The patients’ arm movements were recorded on the tablet and fed into
a computer, which also analyzed the signals coming from the
microelectrodes placed on the area of each patient’s brain controlling
arm and hand movement.

The study showed that the microECoG electrodes could be used to
distinguish brain signals ordering the arm to reach to the right or
left, based on differences such as the power or amplitude of the brain
waves.

The microelectrodes were formed in grid-like arrays embedded in
rubbery clear silicone. The arrays were over parts of the brain
controlling one arm and hand.

The first patient received two identical arrays, each with 16
microelectrodes arranged in a four-by-four square. Individual electrodes
were spaced 1 millimeter apart (about one-25th of an inch). Patient 1
had the ECoG and microECoG implants for a few weeks. The findings
indicated the electrodes were so close that neighboring microelectrodes
picked up the same signals.

So, months later, the second patient received one array containing
about 30 electrodes, each 2 millimeters apart. This patient wore the
electrode for several days.

“We were trying to understand how to get the most information out of
the brain,” says Greger. The study indicates optimal spacing is 2 to 3
millimeters between electrodes, he adds.

Once the researchers develop more refined software to decode brain
signals detected by microECoG in real-time, it will be tested by asking
severe epilepsy patients to control a “virtual reality arm” in a
computer using their thoughts.